Imaging of structures within semiconductors is of interest not only as a research tool but also as a technique of considerable practical importance in the design, fabrication and testing of semiconductor electronic and optoelectronic devices such as integrated circuits. The operating components of such devices are tiny structures having sub-micron features that can be meaningfully viewed only with microscopic techniques.
Considerable difficulty is encountered in viewing a state-of-the-art semiconductor device because plural layers of connective metallization overlie the operative components on the top and a relatively thick silicon layer underlies the components on the bottom.
One approach to microscopically imaging the components is to use optical beam induced current imaging. A focused beam of light at a frequency suitable for exciting electrons from the semiconductor valence band to the conduction band is scanned over the semiconductor chip and the resulting current is measured. From the current generated and the location of the scanning beam, a computer with image processing software can generate an image representative of the features of the device.
Since the top is usually covered with metal, the device is usually scanned through the bottom. The difficulty with this approach, however, is that the beam encounters absorption in passing through the underlying substrate before it reaches the active layer on the upper surface of the device. This reduces the light available for exciting current at the component-rich active layer and superimposes spurious background effects. The result is limitation on the precision with which components can be imaged. Accordingly there is a need for improved methods and apparatus for imaging semiconductor devices.